FIELD OF THE INVENTION
The present invention relates to an encapsulation for electronic components, in particular for components which operate by using surface acoustic waves (SAW components), having a cap which seals component structures on a substrate. The invention also relates to a method for producing the encapsulation.
Encapsulations and housings for electronic components, in particular for SAW components for radio-frequency applications as well, are preferably constructed to be hermetically sealed and include, for example, a metallic cap and a metallic base plate or else a partially metallized carrier for the component system. In the case of SAW components, the term component system in general means a piezo electric substrate having metallic structures mounted on it which form, for example, transducers, resonators or reflectors, as well as electrical connections for those metallic structures. As a rule, the metallic structures are aluminum structures which cannot be passivated. After installation in metal or metal/ceramic housings or encapsulations, short circuits may occur on the metallic structures. Such short circuits are caused by electrically conductive metallic particles. Those particles can become detached from the inside of the cap or from the metallized regions of the component system carrier. Furthermore, the soldering or welding of the cap and base plate or component system carrier may be a source of such Conductive metallic particles, since solder or welding spatter, which cannot be completely avoided, may lead to short circuits on the metallic component structures. Finally, it is also possible for metallic particles to be produced when making contact between wires and the electrical connections of the metallic component structures.
The problem of short circuits resulting from conductive metallic particles of the type mentioned above can be avoided, for example, either by avoiding the production of the particles, by removing existing particles, or by such particles being reliably fixed at safe points.
The particles are predominantly formed on caps and metallic base plates during the production of the caps, for example by friction during rolling or in a thermoforming and stamping tool. Attempts may be made through the use of intensive cleaning processes to keep the number of particles as low as possible or to coat the inside of the caps before assembly, for example with a polymer which binds and fixes the particles. However, polymer coatings are disadvantageous since their application onto defined regions, for example in the case of polygonal caps, is difficult in production terms, complete coating of the inner surface is not possible, because soldered or welded edges and regions of the heat influence zone during soldering or welding must remain free, and space problems caused by relatively thick layers and outgassing from the polymer adversely affect the long-term functionality of the components.
Furthermore, metallic coatings that are composed, for example, of nickel, may be applied electrochemically or non-electrochemically (chemically) in order to fix particles in caps or on metallic base plates. However, it is not possible to achieve complete freedom from particles, even in that way.
In the case of ceramic component system carriers, it is not possible to completely avoid particles of the metallization breaking off or becoming detached, particularly on relatively sharp edges, even when, with that in mind, the component system carrier is constructed optimally, for example by rounding the edges or ending the metallization before the edges.
Furthermore, with the present-day standard, it is impossible to implement wire contacts completely without any friction.
Passivation of the component structures on the substrate through the use of insulating, sufficiently thick layers immediately on the structures is not possible as a rule in the case of high-precision components, since the component characteristics are undesirably influenced even by very thin layers. For example, in the case of SAW components in the form of bandpass filters, that can lead, for example, to a shift in the mid-frequency or to an increase in the bandwidth. Compensation for the change in component characteristics through the use of structures which are trimmed to take account of those aspects is not always possible since, with the present-day standard, it is very difficult to apply thin layers with sufficient layer thickness reproducibility.
It is accordingly an object of the invention to provide an encapsulation for electronic components and a method for producing the encapsulation, which overcome the hereinaforementioned disadvantages of the heretofore-known devices and methods of this general type and which reliably shield metallic component structures against conductive metallic particles in the case of electronic components, in particular in the case of SAW components, in such a way that electrical characteristics, and in the case of SAW components acoustic characteristics as well, are not unacceptably influenced. This is done since, as mentioned above, conductive metallic particles which cause short circuits cannot be completely avoided at an economically acceptable cost and direct passivation of component structures is possible only in exceptional cases.
With the foregoing and other objects in view there is provided, in accordance with the invention, in a component operating with surface acoustic waves (SAW component) and having a substrate and component structures on the substrate, an encapsulation for the component, comprising a cap sealing the component structures on the substrate, the cap being formed by a cover on the substrate and the cover having cutouts formed therein in regions of the component structures for accommodating the component structures.
In accordance with another feature of the invention, the cover includes an upright carrier surrounding the component structures on the substrate and a covering layer applied onto the carrier.
In accordance with a further feature of the invention, the cover is an integral element containing the cutouts.
In accordance with an added feature of the invention, the carrier is a closed frame.
In accordance with an additional feature of the invention, the cover includes supports in addition to the carrier, the supports being disposed on the substrate in regions other than the regions of the component structures.
In accordance with yet another feature of the invention, the cover is bonded, welded or laminated onto the substrate.
In accordance with yet a further feature of the invention, the cover is formed of a material to be structured with a photographic technique.
In accordance with yet an added feature of the invention, the carrier and the supports are formed of a material to be structured with UV light.
In accordance with yet an additional feature of the invention, the carrier and the supports are formed of a photoresist material.
In accordance with again another feature of the invention, the covering layer is formed of a glass material.
In accordance with again a further feature of the invention, the covering layer is formed of a glass ceramic.
In accordance with again an added feature of the invention, the covering layer is formed of a material to be structured by a photographic technique.
In accordance with again an additional feature of the invention, the cover exposes or uncovers electrical connections on the substrate.
In accordance with still another feature of the invention, the cover has openings formed therein for introduction of an acoustic damping compound.
In accordance with still a further feature of the invention, there is provided a plastic sheath disposed over the cover.
In accordance with still an added feature of the invention, the sheath is formed of a plastic film.
With the objects of the invention in view there is also provided a method for producing an encapsulation for a component operating with surface acoustic waves (SAW component) and having a substrate and component structures on the substrate, which comprises forming cutouts in a cover for accommodating the component structures with the cutouts; sealing the component structures on the substrate with the cover to form a cap; producing a plastic film sheath for the covered substrate by dipping, sintering, potting, extrusion coating or press coating using plastic compounds on a base of reactive resins or melted thermoplastics; and placing the sheath over the cover.
In accordance with a concomitant mode of the invention, there is provided a method which comprises producing the cover and the sheath from materials ensuring the cutout, after production of the encapsulation, on the basis of mechanical characteristics of the materials, preferably expansion and shrinkage behavior.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an encapsulation for electronic components and a method for producing the encapsulation, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.